Construction vehicles, such as cranes, excavators and the like, normally comprise an upper frame rotatably mounted on a lower frame and a boom pivotally mounted on the upper frame. Either the front or rear pairs of roadwheels are normally supported under the lower frame by a pair of laterally spaced suspension cylinders pivotally interconnected between an axle housing for each pair of roadwheels and the lower frame. When the vehicle is driven to a remote job site, it is desirable to place the suspension cylinders in a "roading" condition of operation whereby hydraulic fluid may be communicated therebetween to permit the axle housing to oscillate about a pivot means pivotally mounting the axle housing on the lower frame.
Upon the arrival at such job site and during performance of work thereat, it is further desirable to lock-out the performance of work thereat, it is further desirable to lock-out the suspension cylinders to stabilize the vehicle, except when the boom is positioned straight-forward. Conventional lock-out systems of this type normally comprise a separate control valve or in some cases a fluid circuit responsive to activation of the drive motor for the roadwheels which functions to release the cylinders from the lock-out condition of operation and thus place them in the "roading" condition of operation whereby the axle housing is permitted to oscillate for smooth driving purposes. Since the rod end of each suspension cylinder is pivotally connected to the axle housing, an operator cannot detect whether or not the suspension cylinders have been placed in such a "roading" condition of operation.